Drinker very tolerant to start with (3)
I believe the answer is:
sot
'drinker' is the definition.
(sot is a kind of drinker)
'very tolerant to start with' is the wordplay.
'very' becomes 'so' (both can mean 'extremely').
'to start with' suggests taking the first letters.
The first letter of 'tolerant' is 't'.
'so'+'t'='SOT'
(Other definitions for sot that I've seen before include "Habitual drinker" , "Habitual drunkard" , "One drinks" , "Boozer" , "Lush" .)